Because they simply dont need to. I believe the most potent smell a deer leaves in a scrape is from the orbital gland (located on forehead) that makes contact with the licking branch (usually the first thing a deer inspects at a scrape site). Its also possible that the smell from the metatarsals have saturated the inside glands between the hooves and been worked into the dirt, thus having the same affect as physically urinited in the scrape...